1. Field of the Invention
The present invention relates to a call control in an ATM (Asynchronous Transfer Mode) subscriber communication network. More particularly, the present invention relates to a substitution call control system which substitutes a call control from a subscriber terminal in an ATM line concentrator.
2. Description of the Related Art
There are PVC (Permanent Virtual Channel) and SVC (Switched Virtual Channel) in an ATM communication network, and the establishment and release of a SVC connection is controlled.
As protocol for the control of establishment and release of the SVC connection, the following protocols are used. That is, they are:
ITU-T recommendation Q.2110 (to be referred to as SSCOP hereinafter),
ITU-T recommendation Q.2130 (to be referred to as SSCF hereinafter),
ITU-T recommendation Q.2931 and ITU-T recommendation Q.2971,
ATM Forum Technical Committee ATM UNI (User-Network Interface) Specification Version 3.1 (to be referred to as UNI3.1 hereinafter), and
ATM Forum Technical Committee ATM UNI Signaling Specification Version4.0 (to be referred to as UNI4.0 hereinafter) with respect to a signaling layer.
FIG. 1 is a block diagram illustrating a conventional call control system of an ATM communication system. In FIG. 1, an ATM subscriber network 11 is connected with an ATM switching apparatus 20 of an ATM network 10 through a UNI (User-Network Interface) 12. The ATM subscriber network 11 is composed of an ATM subscriber line concentrator 30, and a plurality of network terminators 40a to 40c. The ATM subscriber line concentrator 30 accommodates the plurality of network terminators 40a to 40c. Each of a plurality of subscriber terminals 50a to 50c is connected with a corresponding one of the network terminators 40a to 40c. 
In the conventional call control system, a call control function 100a is installed in the ATM switching apparatus 20 of the ATM network 10. Call control functions 100b and 10c are installed in each of the ATM subscriber line concentrator 30 and the network terminators 40a to 40c in the ATM subscriber network 11. Also, call control functions 100d are installed in the subscriber terminals 50a to 50c. Thus, the flexibility and effective usage of the allocation of network resources in the ATM network 10 and the ATM subscriber network 11 are intended.
FIG. 2 is a diagram illustrating protocols used to realize the conventional call control functions 100a to 100d shown in FIG. 1. In FIG. 2, the ATM switching apparatus 20 in the ATM network 10 is composed of a SDH layer 110a, an ATM layer 111a, an AAL5 layer 112a, an SSCOP layer 113a, an SSCF layer 114a, a signaling layer 115a and an ATM switching apparatus application 130.
The ATM subscriber line concentrator 30 has two layer structures and an ATM subscriber line concentrator application 140. One of the two layer structures is composed of a SDH layer 110b, an ATM layer 111b, an AAL5 layer 112b, an SSCOP layer 113b, an SSCF layer 114b, a signaling layer 115b, and the other is composed of a SDH layer 110c, an ATM layer 111c, an AAL5 layer 112c, an SSCOP layer 113c, an SSCF layer 114c, a signaling layer 115c. 
Each of the network terminators 40a to 40c has two layer structures and a network terminator application 150. One of the two layer structures is composed of a SDH layer 110d, an ATM layer 111d, an AAL5 layer 112d, an SSCOP layer 113d, an SSCF layer 114d, a signaling layer 115d, and the other is composed of a SDH layer 110e, an ATM layer 111e, an AAL5 layer 112e, an SSCOP layer 113e, an SSCF layer 114e, a signaling layer 115e. 
Each of the subscriber terminals 50a to 50c is composed of a SDH layer 110f, an ATM layer 111f, an AAL5 layer 112f, an SSCOP layer 113f, an SSCF layer 114f, a signaling layer 115f and a subscriber-terminal application 160.
As described above, the protocol of each of the call control functions 100a to 100d is composed of:
the physical layer (SDH: Synchronous Digital Hierarchy) 110a to 110f, 
the ATM layer 111a to 111f defined by ITU-T recommendation I.361,
the AAL5 layer (AAL: ATM Adaptation Layer) 112a to 112f defined by ITU-T recommendation I.363,
the SSCOP layer 113a to 113f defined by ITU-T recommendation Q.2110,
the SSCF layer 114a to 114f defined by ITU-T recommendation Q.2130, and
the signaling layer 115a to 115f for executing a call control.
The ATM switching apparatus 20 is provided with an ATM switching apparatus application 130 which controls the call control protocol. The ATM subscriber line concentrator 30 is provided with an ATM subscriber line concentrator application 140 which controls the call control protocol. Each of the network terminators 40a to 40c is provided with a network terminator application 150 and each of the subscriber terminals 50a to 50c is provided with a subscriber terminal application 160.
Next, the operations of the call control functions shown in FIG. 1 will be described.
A call control procedure is executed between the call control functions 10a of the ATM switching apparatus 20 and the call control function 100b of the ATM subscriber line concentrator 30 through a channel 210a of VPI (Virtual Path Identifier)/VCI (Virtual Channel Identifier)=VP identifier/VC identifier=0/5 defined in UNI 3.1 by UNI 12. Thus, the establishment and release of a SVC connection 200a between the ATM switching apparatus 20 and the ATM subscriber line concentrator 30 is controlled.
A call control procedure is executed between the call control function 100b of the ATM subscriber line concentrator 30 and the call control function 10c of the network terminator (40: any of 40a to 40c), using an optional PVC (Permanent Virtual Channel) connection channel 210b as a call control channel. Thus, the establishment and release of the SVC connection 200b is controlled between the ATM subscriber line concentrator 30 and the network terminator 40.
A call control procedure is executed between the call control function 100c of the network terminator 40 and the call control function 100d of the subscriber terminal (50: any of 50a to 50c), using an optional PVC connection channel 210c as a call control channel. Thus, the establishment and release of the SVC connection 200c is controlled between the network terminator 40 and the subscriber terminal 50.
However, there are the following problems in the conventional call control function described above.
The first problem is in that the network terminators 40a to 40c, and the subscriber terminal 50a to 50c are expensive in price. This is because the call control function is installed into each of the ATM subscriber line concentrator 30, the network terminators 40a to 40c, and the subscriber terminal 50a to 50c. Because the call control functions are different depending upon the units, the development cost increases. Also, because the memory capacity is needed for the call control function, each apparatus becomes expensive. As a result, the investment to ATM subscriber network 11 increases.
In the second problem, the maintenance when the specification of the call control function is updated takes time and effort. This is because the call control functions are installed in the ATM subscriber line concentrator 30, the network terminators 40a to 40c, and the subscriber terminal 50a to 50c. In case of the update of the specification of the call control function, a software of the call control function of each of the ATM subscriber line concentrators 30, the network terminators 40a to 40c, and the subscriber terminal 50a to 50c must be updated.
In the third problem, the throughputs of the network terminators 40a to 40c, and the subscriber terminal 50a to 50c are decreased. This is because the SSCOP layer contained in the call control protocol needs to transmit and receive an SSCOP layer control ATM cell in the unit of several seconds for the establishment and maintenance of connection in a link layer.
Also, in a substitution call control procedure which is proposed as a proxy signaling agent (to be referred to as PSA hereinafter) in UNI 4.0 (ATM Forum Technical Committee ATM UNI Signaling Specification Version 4.0), there are two problems. That is, they are where an agent of the substitution call control should be installed in the system, and a communication system between the substitution call control agent and the application of the apparatus is not defined.
In conjunction with the above description, an ATM line concentrator is described in Japanese Laid Open Patent application (JP-A-Heisei 10-65670). In this reference, transmission path terminating circuit (16) to (18), an ATM switch (19), and a signaling terminating circuit (20) are provided. Information for executing signaling is transmitted through the transmission path terminating circuits (16) to (18) using the ATM switch (19) and the signaling terminating circuit (20). The signaling is a procedure to establish a virtual connection channel between the ATM exchanger (1) and one of the subscriber terminals (5) to (8). At this time, the transmission path terminating circuit (16) terminates a physical transmission path (9) which is accommodated by the ATM exchanger (1). Also, the transmission path terminating circuits (17) and (18) terminate physical transmission paths (10) and (11) which are respectively accommodated by the ATM multiplexers (3) and (4) connected to the subscriber terminals (5) to (8), for separating and multiplexing information. Thus, a wide band service can be effectively provided.